The dyeing of textile fibres is a major industry throughout the world and efforts are continually being made to find new and improved materials for this purpose which can provide improvements in terms of factors such as economy, dyeing properties, health and safety and environmental considerations. There is particular interest in developing dyes which can be conveniently and efficiently applied to textile fibres, and which provide intense shades having good fastness properties at reasonable cost.
Coloration of hair, from a social perspective, is a major business, with the numbers of people colouring their hair, both in professional salons and in the home, steadily increasing (http://www.pg.com; http://www.henkel.com). However, people applying colorants to their hair and having their hair coloured are exposed to significant health risks in view of the nature of the materials in use, and it is desirable that alternative natural, non-toxic, non-carcinogenic means of hair coloration are developed in order to minimise any potential hazards to human health. In addition, currently available hair colorants are derived from non-sustainable sources, consuming fossil fuels in their production, and it would be desirable if natural plant-based dyes could instead be utilised in the hair coloration industry.
Permanent hair coloration currently represents at least 80% of the hair coloration market (J. S. Anderson, J. Soc. Dyers Col., 2000, 116, 193). This system relies on the diffusion of uncoloured precursors into the hair where they undergo oxidation reactions to produce the desired colour in situ; these colorants usually last at least 24 shampoos. Permanent oxidative hair coloration typically involves three components, as follows:                (a) the primary intermediate (or “base”), which is usually an ortho- or para-substituted (hydroxy, amine, substituted amine) aromatic amine (e.g. 1-5);        (b) the coupler, which is generally an aromatic compound with at least one (usually two meta-) electron donating group (e.g. 6-13); and        (c) an oxidant, which is almost always hydrogen peroxide (H2O2).        

However, significant concerns exist with regard to the potential toxicological effects of the components which are used, particularly aromatic amines which represent nearly all of the primary intermediates employed, as many of these are suspect carcinogens, tumorgens or mutagens, and may effect reproduction (http://hazard.com/msds/tox). Indeed, it is believed that many of the primary intermediates and couplers may be banned for human use within the foreseeable future. In 2008, the World Health Organisation announced evidence that the hair dye component p-phenylenediamine (PPD; 1) increases the risk of bladder cancer (Cancer Research UK, http://www.cancerhelp.org.uk/help/default.asp?page=4124); PPD is an important component of most hair colorant formulations and, whilst alone it is a suspected carcinogen, in the presence of hydrogen peroxide and the absence of couplers, it may form a trimer—“Bandrowski's base”, as shown in Scheme 1 and discussed by M. Picardo, C. Cannistraci, A. Cristaudo, C. de Luca and B. Santucci (Dermatologica, 1990, 181, 104)—which is known to be mutagenic (B. N. Ames, H. O. Kammen and E. Yamasaki, Proc. Natl. Acad. Sci. USA, 1975, 72, 2423).

PPD and other compounds, such as p-toluenediamine (PTD; 2), resorcinol (6) and ammonia can also provoke severe adverse reactions when used in hair and skin coloration (C. J. Le Coz, C. Lefebvre, F. Keller, E. Grosshans, Arch. Dermatol. 2000, 136, 1515; C. J. Le Coz, Rev. Fr. Aller. Immunol. Clin. 2001, 41, 504, M. Onder, C. A. Atahan, P. Oztas, M. O. Oztas, Int. J. Dermatol. 2001, 40, 577). Allergic contact dermatitis is commonly associated with exposure to p-phenylenediamine, as noted by E. M. Coulter, C. Jenkinson, Y. Wu, J. Farrell, B. Foster, A. Smith, C. McGuire, C. Pease, D. Basketter, C. King, P. S. Friedmann, M. Pirmohamed, B. K. Park and D. J. Naisbett, (J. Investig. Dermatol. 2008, 128, 897). Such suspect compounds are used in most hair colorant formulations and are essential for brown and black shades, which are the most popular colours. Hence, it is desirable, and potentially necessary, to use colorants that pose no significant health hazard; this would necessarily involve a combination of less hazardous synthetic components and greater use of natural compounds.
In addition, many oxidative hair coloration systems rely on the use of alkaline agents, typically ammonia, in order to raise the hair cuticle, thereby allowing the dye precursors to penetrate the hair more easily, whilst also activating hydrogen peroxide. Clearly, improved systems which eliminated the requirement for the use of ammonia would also be desirable.
WO-A-2006/106366 discloses an oxidative hair dye system which eliminates the use of p-phenylenediamine, and which comprises a hair dye, a supposed organometallic compound and an oxidising agent. The supposed organometallic compound is stated to be preferably an organotitanate compound, particularly a tetraalkyl titanate or a titanate chelate. Such compounds are stated to be useful for enhancing the substantivity of topical compositions applied to the body, these compositions comprising one or more cosmetic and/or therapeutic benefit agents, a bonding agent having hydrolysable or exchangeable ligands, and a carrier.
The present inventors have investigated the use of a broader range of metal-containing catalysts in hair dyeing systems in order to avoid the use of potentially harmful materials. The inventors have found that suitable systems may be based on combinations of organic precursors with metal-containing catalysts and oxidising agents. Many of these dye systems also find application in dyeing textile fibre substrates.